1. Field of the Invention.
The present invention relates generally to magneto-optic data recording systems. In particular, the present invention is a method and apparatus for producing write and erase bias fields in a magneto-optic disk drive.
2. Description of the Prior Art.
Magneto-optic data recording technology combines the eraseability features of magnetic data storage systems with the high data storage capacity of optical systems. A 5.25 inch magneto-optic disk can hold up to 600M bytes of information, 1000 or more times the amount of information that a similarly sized magnetic floppy diskette can store. Magneto-optic disks are also transportable and can be transferred between drives. Since the reading, writing and erasing operations are performed with light beams rather than mechanical heads, they have long life, higher reliability, and are relatively immune to physical wear.
The principles of magneto-optic technology are well known. Information is digitally stored at bit positions on a magneto-optic disk. Typical bit positions have a diameter of 0.8 to 2.0 microns. The orientation of the magnetic field at each bit position can be switched between a digital one state in which its north pole is oriented upward, and a second digital zero state in which the magnetic field is reversed and the north pole oriented downward. The orientation of the magnetic field at each bit position is selected by subjecting the bit position to the magnetic field of the appropriate polarity, and heating the bit position of the disk. The magnetic orientation of the bit position is frozen when the disk cools and returns to room temperature.
The magnetic fields of all bit positions in an unwritten disk will generally be oriented north poles down to represent a digital zero. When writing information, the bit positions will be subjected to a write magnetic bias field and heated by a high intensity laser beam. The orientation of the magnetic fields at the written bit positions will then reverse to north poles up. Bit positions are erased by subjecting them to an erase bias field of the opposite polarity, and again heating the bit. The magnetic field orientation at the erased bit positions will then reverse and switch to north pole down.
Data is read from the optical disk using a low-power laser beam. Because of the magneto-optic phenomenon known as the Kerr effect, the polarization of a laser beam impinged upon the bit positions will be rotated as a function of the magnetic orientation of the bit. The polarization of laser beam portions reflected from bit positions on the optical disk is detected by opto-electronic detector circuitry. Signals from the detector circuitry are then processed to determine whether the bit position is representative of a digital one or zero.
A variety of different techniques are currently used to generate the write and erase magnetic bias fields. One technique makes use of an electromagnet positioned adjacent the disk. By reversing of the polarity of the current applied to the electromagnet, both write and erase fields of the required strength can be generated. Although it is relatively inexpensive, and the fields it produces can be quickly switched between the write and erase polarities, significant amounts of power are continuously dissipated by the electromagnet. The heat generated by this power dissipation can have detrimental effects on the disk and other components of the disk drive.
Another known technique makes use of a rotating permanent magnet. Since power is only required to rotate the magnet to its write and erase magnetic field positions, heat dissipation is not a problem. The magnet and rotating mechanism are, however, relatively bulky. As a result, they tend to interfere with tracking and focus servomechanisms, and increase the overall size of the drive. These mechanisms are also relatively expensive.
Yet another technique makes use of two separate permanent magnets, one for producing the write field and the other for producing the erase field. The magnets are mounted to a carriage which is translated or rotated to position the desired magnet adjacent the disk. Power is required only for switching. Although they can be made more compact than the rotating single permanent magnet mechanisms discussed above, they are still relatively bulky and tend to interfere with tracking and focus servos. The translating or rotating mechanism is also relatively slow and expensive.
It is evident that there is a continuing need for improved bias field switching mechanisms for magneto-optic disk drives. In particular, a compact bias field switching mechanism which dissipates little heat and can quickly switch between write and erase bias fields is needed. The system must of course be relatively inexpensive to be commercially viable.